This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:    3GPP third generation partnership project    ANR automatic neighbour relation    ASF autonomous search function    BS base station    CSG closed subscriber group    DL downlink (eNB towards UE)    DRx discontinuous reception    eNB E-UTRAN Node B (evolved Node B)    EPC evolved packet core    E-UTRAN evolved UTRAN (LTE)    HeNB home eNB    HO handover    IMTA international mobile telecommunications association    ITU-R international telecommunication union-radiocommunication sector    LTE long term evolution of UTRAN (E-UTRAN)    LTE-A LTE advanced    MAC medium access control (layer 2, L2)    MM/MME mobility management/mobility management entity    OFDMA orthogonal frequency division multiple access    O&M operations and maintenance    PCI physical cell identifier    PDCP packet data convergence protocol    PHY physical (layer 1, L1)    PSC packet scheduling    RAT radio access technology    Rel release    RLC radio link control    RRC radio resource control    RRM radio resource management    SGW serving gateway    SI system information    SC-FDMA single carrier, frequency division multiple access    UE user equipment, such as a mobile station, mobile node or mobile terminal    UL uplink (UE towards eNB)    UPE user plane entity    UTRAN universal terrestrial radio access network
One modern communication system is known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA). In this system the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V10.4.0 (2011 June), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (EUTRAN); Overall description; Stage 2 (Release 10), referred to for simplicity hereafter as 3GPP TS 36.300. Another specification of interest is 3GPP TS 36.331 V10.2.0 (2011 June) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 10).
FIG. 1A reproduces FIG. 4.1 of 3GPP TS 36.300 and shows the overall architecture of the EUTRAN system (Rel-8). The E-UTRAN system includes eNBs, providing the E-UTRAN user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UEs. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME by means of a S1 MME interface and to a S-GW by means of a S1 interface (MME/S-GW 4). The S1 interface supports a many-to-many relationship between MMEs/S-GWs/UPEs and eNBs.
The eNB hosts the following functions:    functions for RRM: RRC, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);    IP header compression and encryption of the user data stream;    selection of a MME at UE attachment;    routing of User Plane data towards the EPC (MME/S-GW);    scheduling and transmission of paging messages (originated from the MME);    scheduling and transmission of broadcast information (originated from the MME or O&M); and    a measurement and measurement reporting configuration for mobility and scheduling.
Also of particular interest herein are further releases of 3GPP LTE targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. LTE-A is directed toward extending and optimizing the 3GPP LTE Rel-8 radio access technologies to provide higher data rates at lower cost. LTE-A will be a more optimized radio system fulfilling the ITU-R requirements for IMT-Advanced while maintaining backwards compatibility with LTE Rel-8.
A problem relates to small cell discovery and measurement triggering, where by small cells what is generally meant are picocells or HeNB cells. Under current 3GPP assumptions the UE should be able to detect (discover) allowed and earlier visited CSG/hybrid cells using an implementation-specific Autonomous Search Function (ASF). This function is currently specified in a vague manner and the details of this function are left to individual implementations. However, 3GPP is now working towards improving small cell discovery and proximity indications (thus also improving the ASF). Reference in this regard can be made to, for example, 3GPP TSG-RAN Meeting #51, RP-110438, Kansas City, US, Mar. 15-18, 2011, Source: Nokia Siemens Networks, Nokia Corporation, Alcatel-Lucent; Title: New work item proposal for Hetnet Mobility Improvements for LTE, Agenda Item: 14.1.
The challenge that is presented is how should a UE, in the (RRC) connected mode, find a previously discovered and allowed CSG/hybrid cell. Ideally this additional UE task should not lead to a noticeable increase in the signaling load, UE battery power consumption, nor should it have a negative impact on any potentially ongoing data transmission such as, for example, requiring a lower data rate to be used. Under current assumptions the UE is to perform the needed measurements in an implementation specific way, and once a cell has been discovered the network should be notified via a proximity indication message. Based on this message the network configures the UE with an appropriate measurement configuration (on the indicated carrier) and from that time forward then standardized measurement rules are used.
By way of background, reference can be made to Section 10.5.1.2, RRC_CONNECTED, of 3GPP TS 36.300. As is currently specified, while the UE is in the RRC_CONNECTED state the UE performs normal measurement and mobility procedures based on configuration provided by the network. The UE is not required to support manual selection of CSG IDs while in RRC_CONNECTED state.
Handover to a HNB/HeNB follows the framework of UE assisted network controlled handover as described in Section 10.1.2.1. Handover to a HNB/HeNB is different from the normal handover procedure in three aspects:    1. Proximity Estimation: in case the UE is able to determine, using autonomous search procedures, that it is near a CSG or hybrid cell whose CSG ID is in the UE's CSG white list, the UE may provide to the source eNB an indication of proximity. The proximity indication may be used as follows:            If a measurement configuration is not present for the concerned frequency/RAT, the source eNB may configure the UE to perform measurements and reporting for the concerned frequency/RAT.        The source eNB may determine whether to perform other actions related to handover to HNB/HeNBs based on having received a proximity indication (for example, the source eNB may not configure the UE to acquire system information of the HNB/HeNB unless it has received a proximity indication).            2. PSC/PCI Confusion: due to the typical cell size of HNB/HeNBs being much smaller than macro cells, there can be multiple HNBs/HeNBs within the coverage of the source eNB that have the same PSC/PCI. This leads to a condition referred to as PSC/PCI confusion, wherein the source eNB is unable to determine the correct target cell for handover from the PSC/PCI included in the measurement reports from the UE. PSC/PCI confusion is solved by the UE reporting the global cell identity of the target HNB/HeNB.    3. Access Control: if the target cell is a hybrid cell, prioritization of allocated resources may be performed based on the UE's membership status. Access control is done by a two step process, where first the UE reports the membership status based on the CSG ID received from the target cell and the UE's CSG white list, and then the network verifies the reported status.
Mobility from eNB/HeNB to a HeNB CSG/hybrid cell takes place with the S1 Handover procedure. In the following call flow the source cell can be an eNB or a HeNB.
The procedure applies to any scenario where the CSG ID is provided by the UE or provided by the source eNB.
Reference can be made to FIG. 1B, which reproduces FIG. 10.5.1.2-1 “Mobility to HeNBs CSG and hybrid cells” from 3GPP TS 36.300. The enumerated steps and procedures in FIG. 1B are described as follows.    1) The source eNB configures the UE with proximity indication control.    2) The UE sends an “entering” proximity indication when it determines it may be near a cell (based on autonomous search procedures) whose CSG ID is in the UEs CSG white list. The proximity indication includes the RAT and frequency of the cell.    3) if a measurement configuration is not present for the concerned frequency/RAT the source eNB configures the UE with relevant measurement configuration including measurement gaps as needed, so that the UE can perform measurements on the reported RAT and frequency. The network may also use the proximity indication to minimize the requesting of handover preparation information of CSG/hybrid cells by avoiding requesting such information when the UE is not in the geographical area where cells whose CSG IDs are in the UEs CSG White-list are located.    4) The UE sends a measurement report including the PCI (e.g., due to triggered event A3).    5) The source eNB configures the UE to perform SI acquisition and reporting of a particular PCI.    6) The UE performs SI acquisition using autonomous gaps, i.e., the UE may suspend reception and transmission with the source eNB within the limits defined in 3GPP TS 36.133 to acquire the relevant system information from the target HeNB.    7) The UE sends a measurement report including (E-)CGI, TAT, CSG ID and “member/non-member” indication.    8) The source eNB includes the target E-CGI and the CSG ID in the Handover Required message sent to the MME. If the target is a hybrid cell the Cell Access Mode of the target is included.    9) The MME performs UE access control to the CSG cell based on the CSG ID received in the Handover Required message and the stored CSG subscription data for the UE. If the access control procedure fails, the MME ends the handover procedure by replying with the Handover Preparation Failure message. If the Cell Access Mode is present, the MME determines the CSG Membership Status of the UE handing over to the hybrid cell and includes it in the Handover Request message.    10-11) The MME sends the Handover Request message to the target HeNB including the target CSG ID received in the Handover Required message. If the target is a hybrid cell the CSG Membership Status will be included in the Handover Request message.    12) The target HeNB verifies that the CSG ID received in the Handover Request message matches the CSG ID broadcast in the target cell and if such validation is successful it allocates appropriate resources. UE prioritization may also be applied if the CSG Membership Status indicates that the UE is a member.    13-14) The target HeNB sends the Handover Request Acknowledge message to the MME via the HeNB GW if present.    15) The MME sends the Handover Command message to the source eNB.    16) The source eNB transmits the Handover Command (RRC Connection Reconfiguration message including mobility control information) to the UE.
NOTE: Steps 1-9, 15 and 16 also apply to inter-RAT mobility from LTE to HNB.
After sending an “entering” proximity indication (step 2), if the UE determines that it is no longer near a cell whose CSG ID is in the UE's CSG white list, the UE sends a “leaving” proximity indication to the source eNB. Upon reception of this indication, the source eNB may reconfigure the UE to stop measurements on the reported RAT and frequency.
In the above procedure as currently specified in 3GPP TS 36.300 steps 2 and 3 may not be performed in case the UE has not previously visited the HeNB, e.g., when the UE first visits a hybrid cell.
The PCI confusion is said to be resolved by steps 5, 6 and 7. The source eNB can request SI acquisition and reporting for any PCI, not limited to PSCs/PCIs of CSG or hybrid cells.